I see atheism as being internally inconsistent. I don't think it's rational to hold an atheistic viewpoints. I think that when you examine the evidence that you'll conclude that there must be a god ... [Atheism is] the act of denial of God. - Matt Slick - Atheist Experience #592 @ 1:06:00

refusal to admit the truth or reality (as of a statement or charge) (2) : assertion that an allegation is false

refusal to acknowledge a person or a thing : disavowal

the opposing by the defendant of an allegation of the opposite party in a lawsuit

self-denial

negation in logic

a psychological defense mechanism in which confrontation with a personal problem or with reality is avoided by denying the existence of the problem or reality

The bold entries are the definitions that would seem to be relevant to the discussion. Overall, the accusation is this:

God is proven

The atheist, for whatever reason, doesn't want to admit to the obvious truth

The atheist is in denial

But god is not proven, so it is unbelief in the unproven. Which is not denial at all. It is merely disbelief in the unproven.

Counter Apologetics

Besides the strawman definition of atheism, in order for the accusation to hold true, the assertion that sufficient evidence exists to support the God claim must be demonstrated. The #1 most consistently and demonstrably effective means at precisely and accurately learning about reality, and how it works, is the scientific method. Arguably, no other method even remotely comes close. Thus, by default, the scientific method is employed. Presumably, one would use the most effective tool for the job, not the least.

In order for the evidence to be sufficient, several (minimum) requirements must be met:

Failure to meet these requirements means the evidence is invalidated, and can be rationally discarded, until the problems are addressed and resolved. The accusation that evidence exists that indicates a god is invalid, and thus, there's nothing to be "in denial' about.